1. Field of the Invention
The present invention relates to an intermediate telephoto lens system, having an angle-of-view of approximately 25°, for use in a digital single lens reflex (SLR) camera.
2. Description of the Prior Art
An intermediate telephoto lens system has generally been defined as a lens system having a focal-length range from 75 mm to 135 mm with respect to the 135 (35 mm) film format.
Particularly, an intermediate telephoto lens system with an angle-of-view of 25° (equivalent to a focal length of a little more than 100 mm) has a long-lasting demand (popularity) for portrait use with a single focal length; and for use in a digital SLR camera, further miniaturization of an intermediate telephoto lens system in the optical axis direction has been required (i.e., a thinner lens system).
The size of an imaging device for a digital SLR camera is generally smaller than that of a film-frame of the 135 (35 mm) film format. However, in a digital SLR camera, the flange back length is arranged to be the same as that of a silver-halide film SLR camera so that a photographing lens system designed for a silver-halide film SLR camera can be used with a digital SLR camera, i.e., compatibility between a silver-halide film SLR camera and a digital SLR camera.
Consequently, an exchangeable photographing lens system designed for a digital SLR camera, compared with an exchangeable photographing lens system designed for a silver-halide film SLR camera, inevitably faces a problem to be solved, i.e., the flange back length has to be maintained longer, while the focal length has to be made shorter in proportion to the size of an imaging device.
Note that redesigning conventional photographing lenses system to adjust the same with the size of an imaging device (CCD) cannot solve the above problem, because according to such a simple re-scaling, when the focal length becomes shorter, the back focal distance also becomes shorter.
Moreover, conventional interchangeable lens systems have not sufficiently considered further miniaturization thereof in the optical axis direction (i.e., a thinner lens system).
The present invention is to provide a thinner intermediate telephoto lens system of a modified-Gauss-type in which higher optical performance is attained by (i) adequately distributing refractive power over each lens group, and by (ii) selecting appropriate glass lens materials for the lens elements.